Live-tracking rhizobial switch from rhizosphere to plant-confined colonization modes

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Legumes establish beneficial nitrogen-fixing symbioses with rhizobia bacteria that supply nitrogen for the plant within specialized root organs, known as nodules. Early stages of this interaction require crucial molecular exchanges in the rhizosphere before the host plant reprograms for bacterial root entry. While most rhizobia species infect the roots of their host via a newly formed root hair intracellular apoplastic compartment called infection thread (IT), others colonize their host plants via alternative and less studied intercellular infection modes. In both cases, rhizobia must switch from a rhizospheric to an infecting state where they become confined within plant apoplastic compartments. Up to now, it has been technically challenging to study bacterial reprogramming within plant-confined environments during early infection, as these events are restricted to a few root cells. Live-Switch is an innovative project aimed at addressing the crucial but largely unexplored question of how rhizobia cope with the drastic transition from free living to infective within-host state. The project will take advantage of Medicago and Lotus model legume systems establishing intracellular or intercellular infection modes, to decipher at the single cell level how rhizobia cells reprogram during the switch from a free-living to a host-confined environment. The project will combine live cell imaging and mutant analyses of both bacteria and plant partners to dissect the dynamics of rhizobial signal production, cell proliferation and motility and the cross talk with the plant partner. The integration of data in a final mathematical model will permit a better understanding of the dynamics of spreading and proliferation of rhizobia in the in planta IT compartment. Live-Switch relies on an ideal integrated and collaborative frame between the French and German groups, that with their solid and complementary expertise in molecular and cell biology of both plant and bacteria partners, will be able to address the exciting question of how dynamic environments shape the in vivo switch of bacterial reprograming to in planta lifestyles.